Prior art blends containing two polyethylene components, one component being a linear low density polyethylene and one component being a low density polyethylene, such as Dow Chemical Company's Linear Low Density Polyethylene 3010, are useful in certain extrusion coated structures. Examples of such structures are flexible polymeric film/paper packages for foods, and metallized polymeric film balloons. The structures comprise a substrate extrusion coated with the prior art blends of polyethylene. The extrusion coatings serve as heat seal media and as barriers to protect the contents of a package, e.g., food, from outside contamination, or to retain the contents, e.g., helium in a coated and sealed balloon. The linear low density polyethylene component of the prior art two-component blend is used to provide the strong heat seal strengths associated with each application's structure.
The substrates (e.g., polymeric films or metal films used in the articles described above) do not readily accept the prior art two-component polyethylene blends to permit good bonds between the extrusion coatings and the films. Therefore, the substrates are usually primed prior to extrusion coating with a water-soluble primer. Typically, a primer comprising polyethyleneimine is used. A particularly preferred polyethyleneimine primer is MICA Corporation's A-131X formulation. The primer readily wets the surfaces of the substrate and adheres thereto. The prior art polyethylene blends are then applied to the polyethyleneimine primer surface by an extrusion coating process well-known in the art. This extrusion coating application of the prior art polyethylene blends can, with sufficient curing time, ultimately permit a satisfactory bond between the prior art polyethylene blend and the polymeric films.
However, there is a problem associated with this extrusion coating process involving the prior art polyethylene blends. The prior art polyethylene blends, such as those claimed in U.S. Pat. No. 4,339,507, when extrusion coated against the polyethyleneimine primer, do not provide a satisfactory immediate bond. Instead, the prior art blends, containing a linear low density polyethylene component made by copolymerizing octene with an ethylene homopolymer, require a curing time to reach a minimally acceptable bond strength, usually considered to be about 450-g/inch (177-g/cm). Such curing times usually range from about 15-minutes to 2-hours. During these curing times, the extrusion coating operator does not know if the extrusion coated products, coated with the prior art polyethylene blends, will exhibit satisfactory structural integrity, i.e., a satisfactory bond strength between the substrate and the coating in the presence of the thin polyethyleneimine primer layer between the coating and the substrate.
Thus, during these curing times the operator continues to extrusion coat a product which may prove to be defective and unusable. Most extrusion coating production processes operate at line speeds in excess of 800-fpm (245-m/min). When the required curing times of about 15-120 minutes are considered in such an operation, a significant amount of extrusion coated product can be wasted with resulting economic losses to the extrusion coating operation.
Thus, there is a need for an extrusion coating polyethylene blend containing a linear low density polyethylene component that would give quick bond to a polyethyleneimine primed substrate, preferably about an order of magnitude faster bond. Such a blend would provide an extrusion coating operator with immediate knowledge as to the bond strengths of the product without the long waiting time required of the prior art blends.